Another addendum: I'm sure you know that if you use regular craft glue, it will be wet and take awhile to dry. After smearing your glue and adding and tapping off the excess sand, lay the card flat, place a piece of copy paper over it and then lay something heavy on top (make sure that there are no wet spots of glue for the paper to get stuck). This will assure the card dries flat.

Just experiment on scrap paper that will be the size of the area you need for your card.